The invention is directed to a current transformer for alternating current, particularly mains alternating current, having dc parts, composed of at least one transformer core with a primary winding and at least one secondary winding to which a load resistor is connected in parallel and which terminates the secondary circuit in low-impedance fashion.
Such current transformers have been known for a long time. The current transformers transform a primary current onto a secondary current in relationship to the numbers of turns between primary and secondary winding, this secondary current then being acquired potential-free at the load resistor by a measuring instrument or a digital evaluation circuit. The range of current can, for example, be 100 A primary onto 50 mA secondary, and the secondary range of current can be of a standardized size. FIG. 1 shows the schematic circuit of a such a current transformer 1. The primary winding 2, which carries a current i.sub.prim to be measured, and a secondary winding 3, which carries the test current i.sub.sec are located on a transformer core 4 that can be constructed of tape cores similar to power transformers. The secondary current i.sub.sec is automatically established such that, ideally, the ampere turns at the primary and secondary side are of the same size and oppositely directed, for example i.sub.prim =600 A and turns n.sub.prim =2 at the primary side and i.sub.sec =5 A and turns n.sub.sec =240 at the secondary side. With a phase shift of 180.degree. between primary current and secondary current. This derives from Lenz's Law, according to which the induction current is always certain to be established such that it attempts to prevent the driving cause.
The secondary winding is terminated low-impedance via a load resistor R.sub.B 5, i.e. the load resistor R.sub.B 5 is far, far smaller than the impedance of the secondary winding, i.e. R.sub.B &lt;&lt;.OMEGA.L. The magnetic fields that are generated by the two windings in the core--and this is the special feature of the current transformer--are of nearly the same size and directed opposite one another at any point in time. Only an extremely small magnetic flux is thus generated in the transformer core, this inducing a secondary current that just maintains the test current through the load resistor R.sub.B 5. Relative to the strength of the magnetic field emanating from the primary current, thus, the transformer core 4 is driven only very slightly.
Due to the eddy current losses and the remagnetization losses in the transformer core, losses in the windings and the load resistor, the ideal case is not completely achieved. What is understood by the quality factor of the current transformer is the ratio of the loss resistance R.sub.v and the impedance of the secondary coil .OMEGA.L. The following relationships apply to the quality factor of the current transformer and should be optimally small: ##EQU1## whereby tan .delta. denotes the phase shift between i.sub.prim and i.sub.sec, H denotes amplitude of the magnetic field strength, B denotes amplitude of the magnetic field density B, R.sub.v denotes the loss resistance of the current transformer in which all loss mechanisms are combined and denotes the relationship between the magnetic drive of the transformer core to the drive field under the term at the right side of Equation (2).
Accordingly, the secondary current i.sub.sec exhibits a small phase shift relative to the driving current i.sub.prim and the amplitude of the magnetic flux density in the transformer core is significantly lower than given an exclusive drive by only the primary current. Typical values for the factor R.sub.v /.OMEGA.L lie between 1/100 and 1/500.
The magnetic flux density B in the transformer core exhibits a phase shift of nearly -90.degree. relative to the drive to the magnetic field or, respectively, the primary current. It thus has maximum values respectively close to the zero-axis crossings of primary current and secondary current.
These maximum values dare not reach the saturation flux density B.sub.sat of the core material. The current range that can be covered by a current transformer is defined by Equation (2) and the material constant B.sub.sat. The above explanations are illustrated by FIG. 2.
Accordingly, the current transformers of the type species initially cited only function given nearly purely symmetrical alternating current. A dc component that can occur due to rectifying component parts in the primary circuit places the transformer core into magnetic saturation very quickly. The current transformer is then no longer functional.
This shall be explained below with reference to an example:
When a diode is situated in the primary circuit, then a pure half-wave rectification occurs thereat. The dc component of this form of current amounts to i.sub. ==1/.pi.i. A current transformer that is designed for an alternating current amplitude of 100 A, accordingly, can already no longer work cleanly given a half-wave current with an amplitude of 1 A.
However, it is precisely a high dc tolerance that is demanded of current transformers that are to be utilized in energy meters. This demand was hitherto been taken into account in that the transformer cores employed were very highly over-dimensioned and, over and above this, were also potentially connected to a primary shunt, which sees to it that only a part of the primary current is conducted through the transformer core.